1. Field of the Invention
The present invention relates to a method for controlling pressure during image development, and more particularly, to a method for controlling pressure applied to a press roller unit in an image forming apparatus in which a photosensitive sheet coated with microcapsules containing photosetting materials and coloring dyes is exposed to an image forming light and superimposed on an image receiving sheet so that the overlaying sheets are pressed together so as to form an image on the image receiving sheet.
2. Description of the Prior Art
When a photosensitive sheet coated with microcapsules containing photosetting materials and coloring dyes is exposed to an image forming light such as a reflected light from a document, the microcapsules are selectively caused to harden to form a latent image on the photosensitive sheet. An image receiving sheet is then superimposed over the latent image on the photosensitive sheet, and the two sheets are pressed together, thereby causing unhardened microcapsules to rupture allowing the coloring dyes contained therein to flow out to form a colored image on the image receiving sheet. The photosensitive sheet is normally mounted on a roll, from which the sheet is drawn out by the leading edge thereof for image forming exposure. When the photosensitive sheet is exposed to light, unexposed areas remain between the current latent image and the preceding latent image (inter-image areas) on the photosensitive sheet. Since these areas are left unexposed to light, the microcapsules in these areas rupture under pressure, thereby causing an unexpected flow-out of the coloring dyes. To prevent this, there is a proposal for applying pressure only to exposed areas. To achieve this, the press roller unit is designed to receive pressure only when it is applied.
However, the problem is that since the pressure apply/release control for the press rollers is performed on the basis of the size of the image receiving sheet, edge portions of the image receiving sheet may be stained with microcapsules that are unexpectedly ruptured.
More specifically, the known method employs a sensor for detecting an image sheet, the sensor being disposed in place before the press roller, and pressure is applied upon lapsing of a prescribed period of time after the sensor detects the leading edge of the image receiving sheet, and is released upon the lapsing of another prescribed time after the sensor detects the trailing edge of the image receiving sheet. However, when the latent image size is relatively smaller than the image receiving sheets size (particularly when the image is contracted to a smaller size), the trailing edge of the image receiving sheet extends beyond the latent image area, thereby causing an unnecessary portion of microcapsules to rupture. This stains the image receiving sheet with the ruptured coloring agent, and spoils it.